It is well known that the efficiency of supported catalyst systems is often related to the surface area on the support. This is especially true for systems using precious metal catalysts or other expensive catalysts. The greater the surface area, the more catalytic material is exposed to the reactants and the less time and catalytic material is needed to maintain a high rate of productivity.
Alumina (Al2O3) is a well-known support for many catalyst systems. It is also well known that alumina has a number of crystalline phases such as alpha-alumina (often noted as α-alumina or α-Al2O3), gamma-alumina (often noted as γ-alumina or γ-Al2O3) as well as a myriad of alumina polymorphs. Gamma-Al2O3 is a particularly important inorganic oxide refractory of widespread technological importance in the field of catalysis, often serving as a catalyst support. Gamma-Al2O3 is an exceptionally good choice for catalytic applications because of a defect spinel crystal lattice that imparts to it a structure that is both open and capable of high surface area. Moreover, the defect spinel structure has vacant cation sites giving the gamma-alumina some unique properties. Gamma-alumina constitutes a part of the series known as the activated, transition aluminas, so-called because it is one of a series of aluminas that can undergo transition to different polymorphs. Santos et al. (Materials Research, 2000; vol. 3 (4), pp. 104-114) disclosed the different standard transition aluminas using Electron Microscopy studies, whereas Zhou et al. (Acta Cryst., 1991, vol. B47, pp. 617-630) and Cai et al. (Phys. Rev. Lett., 2002, vol. 89, pp. 235501) described the mechanism of the transformation of gamma-alumina to theta-alumina.
The oxides of aluminum and the corresponding hydrates, can be classified according to the arrangement of the crystal lattice. Some transitions within a series are known; for example, low-temperature dehydration of an alumina trihydrate (gibbsite, Al(OH)3) above 100° C. with the presence of steam provides an alumina monohydrate (boehmite, AlO(OH)). Continued dehydration at temperatures above 450° C. leads to the transformation from boehmite to γ-Al2O3. Further heating may result in a slow and continuous loss of surface area and a slow conversion to other polymorphs of alumina having much lower surface areas. Thus, when gamma-alumina is heated to high temperatures, the structure of the atoms collapses such that the surface area decreases substantially. Higher temperature treatment above 1100° C. ultimately provides α-Al2O3, a denser, harder oxide of aluminum often used in abrasives and refractories. While alpha-alumina has the lowest surface area, it is the most stable of the aluminas at high temperatures. Unfortunately, the structure of alpha-alumina is less well suited to certain catalytic applications because of a closed crystal lattice, which imparts a relatively low surface area to the alpha-alumina particles.
Alumina is ubiquitous as supports and/or catalysts for many heterogeneous catalytic processes. Some of these catalytic processes occur under conditions of high temperature, high pressure and/or high water vapor pressure. The prolonged exposure to high temperature typically up to 1,000° C., combined with a significant amount of oxygen and sometimes steam can result in catalyst deactivation by support sintering. The sintering of alumina has been widely reported in the literature (see for example Thevenin et al, Applied Catalysis A: General, 2001, vol. 212, pp. 189-197) and the phase transformation of alumina due to an increase in operating temperature is usually accompanied by a sharp decrease in surface area. In order to prevent this deactivation phenomenom, various attempts have been made to stabilize the alumina support against thermal deactivation (see Beguin et al., Journal of Catalysis, 1991, vol. 127, pp. 595-604; Chen et al., Applied Catalysis A: General, 2001, vol. 205, pp. 159-172).
For example, it is well known that adding lanthanum to alumina, a process also known as La-doping, can stabilize the alumina structure. Specifically, U.S. Pat. No. 6,255,358 discloses a catalyst comprising a gamma-alumina support doped with an amount of lanthanum oxide, barium oxide, or a combination thereof effective for increasing the thermal stability of the catalyst. The patent discloses a catalyst comprising per 100 parts by weight of the support from about 10-70 parts by weight cobalt and optional components, including from about 0.5 to 8 parts by weight lanthana. Similarly, U.S. Pat. No. 5,837,634 discloses a process for preparing a stabilized alumina, e.g., gamma alumina, of enhanced resistance to high temperature surface area loss such as by the addition of lanthana to a precursor boehmite alumina. In an example, a mixture of boehmite alumina, nitric acid, and stabilizers such as lanthanum nitrate was dispersed and the mixture aged for 4 hours at 350° F. Subsequently, the formed powder was calcined for 3 hours at 1200° C.
In general, the prior art has focused on the stabilization of alumina by using a small amount of lanthana, typically below 10%, and in most practices between 1-6 wt. %. In “Characterization of lanthana/alumina composite oxides,” S. Subramanian et al., Journal of Molecular Catalysis, Volume 69, 1991, pages 235-245, lanthana/alumina composite oxides were formed. It was found that as the lanthana weight loading increased, the surface area of the lanthana dispersed in the composite oxide also increased and reached a plateau at 8% La2O3 loading. It was also found that the total BET surface area of the composite oxide decreased sharply as the lanthana loading increased above 8%. The composite oxides were prepared by the incipient wetness procedure in which the alumina was impregnated with lanthanum nitrate hexahydrate and the precursors dried and then calcined at 600° C. for 16 hours.
It has also been reported that surface perovskite-species on alumina can be formed by doping the alumina with small amounts of lanthanum. Thus, in “The Influence of High Partial Steam Pressures on the Sintering of Lanthanum Oxide-Doped Gamma Alumina,” H. Schaper et al., Applied Catalysis, 1984, Volume 9, pages 129-132, experiments were conducted by doping gamma alumina with 0-5 mol % lanthanum oxide. In all the lanthanum oxide-promoted samples, lanthanum aluminate (LaAlO3) lines were observed. The addition of 4-5 mol % of lanthanum oxide drastically decreased the surface area loss of gamma alumina at the high partial steam pressures.
For most of the lanthana-doped alumina compositions, the lanthanum is in the form of lanthanum oxide. In “Dispersion Studies on the System La2O3Y-Al2O3,” M. Bettman et al., Journal of Catalysis, Volume 117, 1989, pages 447-454, alumina samples with different lanthanum concentrations were produced by impregnation with aqueous lanthanum nitrate, followed by calcination at various temperatures. It was found that up to a concentration of 8.5 μmol La/m2, the lanthana was in the form of a 2-dimensional overlayer, invisible by XRD. For greater lanthana concentrations, the excess lanthana formed crystalline oxides detectable by XRD. In samples calcined to 650° C., the crystalline phase was cubic lanthanum oxide. After calcination at 800° C., the lanthana reacted to form the lanthanum aluminate, LaAlO3.
The formation of perovskite, i.e., LaAlO3, is often treated as a minor intransient species formed at very high temperatures, typically above 1100° C., and it is generally believed that the reaction of a small amount of lanthanum with alumina at high temperatures leads to the formation of lanthanum hexa-aluminate, or beta-alumina, U.S. 2004/0138060A1, published Jul. 15, 2004.
Destabilization of the support is not the sole cause of catalyst deactivation at high temperature. Stabilizing the catalytically active species on a thermally stable support is also needed. When an active species is supported on an oxide support, solid state reactions between the active species and the oxide support can take place at high temperature, creating some instability. That is why Machida et al. (Journal of Catalysis, 1989, vol. 120, pp. 377-386) proposed the introduction of cations of active species through direct substitution in the lattice site of hexaaluminates in order to suppress the deterioration originating from the solid state reaction between the active species and the oxide support. These cation-substituted hexaaluminates showed excellent surface area retention and high catalytic activity (see the hexaaluminate examples with Sr, La, Mn combinations in Machida et al., Journal of Catalysis, 1990, vol. 123, pp. 477-485). Therefore the preparation procedure for high temperature catalysts is critical for thermal stability and acceptable surface area.
It has long been a desire in the catalyst support arts to have a form of alumina that has high surface area like gamma-alumina and stability at high temperature like alpha-alumina. Such a catalyst support would have many uses.
Perovskite catalysts are known to have good stability in a wide variety of chemical environments. Perovskite compositions are nominally designated as ABO3. For perovskites containing rare earth and transition metals, A represents a rare earth metal, such as lanthanum, neodymium, cerium or the like, and B represents a transition metal such as cobalt, iron, nickel or the like. It is known in the art that perovskite-type materials are useful for the catalytic oxidation and reduction reactions associated with the control of automotive exhaust emissions. Several techniques have been used to produce perovskite-type catalyst materials for the treatment of exhaust gases from internal combustion engines. The following patents describe such materials and techniques in the three-way catalytic application: U.S. Pat. Nos. 3,865,752; 3,865,923; 3,884,837; 3,897,367; 3,929,670; 4,001,371; 4,049,583, 4,107,163; 4,126,580; 5,318,937. In particular, Remeika in U.S. Pat. No. 3,865,752 describes the use of perovskite phases incorporating Cr or Mn on the B-site of the structure showing high catalytic activity. Lauder teaches in U.S. Pat. No. 4,049,583 (and U.S. Pat. No. 3,897,367) the formation of single-phase perovskite materials showing good activity for CO oxidation and NO reduction. Tabata in U.S. Pat. No. 4,748,143 teaches the production of single-phase perovskite oxidation catalysts where the surface atomic ratio of the mixed rare earth elements and the transition metal is in the range of 1.0:1.0 to 1.1:1.0. The rare-earth component can be introduced using a mixed rare-earth source called “Lex 70” which has a very low Ce content. Tabata further teaches in U.S. Pat. No. 5,185,311 the support of Pd/Fe by perovskites, together with bulk ceria and alumina, as an oxidation catalyst. The perovskite is comprised of rare earths on the A-site and transition metals on the B-site in the ratio 1:1.
In addition to these patents there are numerous studies reported in the scientific literature relating to the fabrication and application of perovskite-type oxide materials in the treatment of internal combustion exhaust emissions. These references include Marcilly et al., J. Am. Ceram. Soc., 53 (1970) 56; Tseung et al., J. Mater. Sci., 5 (1970) 604; Libby, Science, 171 (1971) 449; Voorhoeve et al., Science, 177 (1972) 353; Voorhoeve et al., Science, 180 (1973); Johnson et al., Thermochimica Acta, 7 (1973) 303; Voorhoeve et al., Mat. Res. Bull., 9 (1974) 655; Johnson et al., Ceramic Bulletin, 55 (1976) 520; Voorhoeve et al., Science, 195 (1977) 827; Baythoun et al., J. Mat. Sci., 17 (1982) 2757; Chakraborty et al., J. Mat. Res., 9 (1994) 986. Much of this literature and the patent literature frequently mention that the A-site of the perovskite compound can be occupied by any one of a number of lanthanide elements (e.g., Sakaguchi et al., Electrochimica Acta, 35 (1990) 65). In all these cases, the preparation of the final compound utilizes a single lanthanide, e.g., La2O3. Meadowcroft, in Nature, 226 (1970) 847, refers to the possibility of using a mixed lanthanide source for the preparation of a low-cost perovskite material for use in an oxygen evolution/reduction electrode. U.S. Pat. No. 4,748,143 refers to the use of an ore containing a plurality of rare-earth elements in the form of oxides for making oxidation catalysts.
While perovskites are quite stable under harsh environments such as high temperatures, perovskites are not porous and, thus, have low surface area. Accordingly, for use as catalytic supports, in particular, such materials have not found wide applications.